[CC-COUPLES] couples sit next to each other

[CC-COUPLES] couples sit next to each other:

You\ (n \ le5 \ times10 ^ 5) \) sat in a circle with friends \ (2N. At the beginning, people numbered \ (I \) sat in the \ (I \) seat. Each time, two adjacent people can exchange seats. How many exchanges are required for each pair of seats adjacent to each other?

Ideas:

The answer is the logarithm of a pair of friends whose distance is the sum of two people-"crossover. Tree array maintenance.

Time Complexity \ (\ mathcal O (N \ log n )\).

Source code:

#include<cstdio>#include<cctype>#include<algorithm>inline int getint() {    register char ch;    while(!isdigit(ch=getchar()));    register int x=ch^'0';    while(isdigit(ch=getchar())) x=(((x<<2)+x)<<1)+(ch^'0');    return x;}typedef long long int64;const int N=5e5+1;int n,a[N*2],pos[N][2],cnt[N];class FenwickTree {    private:        int val[N*2];        int lowbit(const int &x) const {            return x&-x;        }    public:        void modify(int p,const int &x) {            for(;p<=n*2;p+=lowbit(p)) {                val[p]+=x;            }        }        int query(int p) const {            int ret=0;            for(;p;p-=lowbit(p)) {                ret+=val[p];            }            return ret;        }        int query(const int &l,const int &r) const {            return query(r)-query(l-1);        }};FenwickTree t;int main() {    for(register int T=getint();T;T--) {        n=getint();        std::fill(&cnt[1],&cnt[n]+1,0);        for(register int i=1;i<=n*2;i++) {            const int &x=a[i]=getint();            pos[x][cnt[x]++]=i;        }        int64 ans=0;        for(register int i=1;i<=n;i++) {            ans+=std::min(pos[i][1]-pos[i][0],n*2+pos[i][0]-pos[i][1])-1;        }        for(register int i=1;i<=n*2;i++) {            const int &x=a[i];            if(i==pos[x][0]) {                t.modify(i,1);            }            if(i==pos[x][1]) {                t.modify(pos[x][0],-1);                ans-=t.query(pos[x][0],i);            }        }        printf("%lld\n",ans);    }    return 0;}

[CC-COUPLES] couples sit next to each other